1. The Field of the Invention
The invention relates to methods for detecting rare earth minerals. More particularly, the present invention provides quick and accurate methods for detecting the presence of rare earth minerals which can be performed in the field.
2. Technology Review
The rare earth elements, or lanthanides, are the fifteen elements in the periodic table from atomic number 57 through 71. Although not members of the lanthanide series, yttrium (atomic number 39) and scandium (atomic number 21) are often grouped with the rare earth elements because they frequently occur with them in nature and have similar chemical properties.
As a group, the rare earth elements are more abundant in the earth's crust than nickel or copper; cerium alone is more abundant than tin; neodymium and lanthanum are more abundant than lead. Although elements of the lanthanide series are historically called "rare earths", they are neither rare nor earths ("earths" being a term used for oxides in the century of early lanthanide discovery).
One of the most important applications of rare earths is in catalytic activities. Petroleum refineries use a lanthanum-rich rare earth mixture to increase the yield of gasoline and other aromatics from heavy crude oils.
Another significant application of rare earths is in the field of high temperature superconductors. Certain lanthanum- and yttrium-containing compounds are known to possess high temperature superconducting properties. In addition, many other rare earth-containing compounds are being examined for high temperature super conductivity.
Mischmetal, produced by the electrolysis of anhydrous mixed rare earth chlorides, has applications in the iron and steel and the cigarette lighter flint industries. In the iron and steel industry, the physical and rolling properties of the metal are improved by the use of mischmetal. Rare earth treated, high strength, low alloy steels are being increasingly used in the automobile industry as structural components and in lightweight steel applications.
Rare earth metals are used in the manufacture of permanent magnets. These high strength permanent magnets result in lighter, smaller, and more energy efficient electric motors and generators.
The red phosphor component in all color television sets uses europium and yttrium oxides. In addition, color television face plates contain neodymium to enhance the picture brightness and contrast. X-ray intensification screens containing lanthanum or gadolinium based phosphors reduce patient diagnostic exposure times by more than half.
Other important applications of the rare earths include: ceramics and optics (including polishing compounds and glass additives), electronics, nuclear energy, lighting and lasers. New uses for rare earth minerals in high technology industries are being discovered every year.
The rare earth elements and yttrium are essential constituents in more than 100 rare earth minerals. However, only a few rare earth minerals occur in sufficient concentration to qualify as ore. Monazite, bastnasite, xenotime, euxenite, samarskite, and allanite are a few examples of the many rare earth minerals. Of these rare earth minerals, bastnasite is of particular importance in the United States. The world's largest producer of bastnasite is the mine at Mountain Pass, Calif. Although rare earth elements are not as rare as once believed, minable deposits of rare earth minerals are rare.
Identification of rare earth elements is difficult, and identification of rare earth minerals is also difficult. One explanation for the difficulty of chemically identifying rare earths lies in the fact that virtually all of the rare earth elements are characterized by a +3 oxidation state. The lanthanides are characterized by the gradual filling of the 4f subshell. The relative energies of the 5d and the 4f orbitals are very similar and sensitive to the occupancy of these orbitals. The universal preference for the +3 oxidation state together with the notable similarity in size led to great difficulties in separating rare earth elements prior to the development of chromatographic methods.
In addition, as a consequence of the poor shielding of the 4f electrons, there is a steady increase in effective nuclear charge and concomitant reduction in size. The ionic radius of the trivalent ion from lanthanum to lutetium gradually decreases in size, an effect known as the lanthanide contraction. As a result, the heavier rare earth elements have an ionic radius similar to much lighter elements, such as yttrium.
Since rare earth minerals are not readily identifiable, especially if they are not radioactive, it is fairly likely that commercially important deposits of rare earth minerals exist in many parts of the nation. Of the thirty important rare earth minerals, only half are radioactive. However, they are usually associated with minerals which are radioactive, so radioactivity is an important feature in identifying rare earth minerals. Radioactivity may aid in the discovery of rare earth deposits, but it is also an expensive nuisance, as safeguards must be taken and regulatory standards maintained while handling concentrates.
Rare earth minerals are often found disseminated in potash-rich igneous rocks, and they are commonly in complex pegmatite dikes along with many other rare minerals and gemstones. Rare earth minerals like bastnasite are occasionally found in large quantities in an unusual marble-like intrusive igneous rock known as "carbonatite." Sometimes, rare earth minerals are found disseminated in metamorphic rocks such as gneiss and schist. They are also found in certain veins and along fault zones in association with hematite, barite, fluorite, and other minerals. The presence of smoke quartz and radioactive halos is sometimes a clue to rare earths, since they are usually associated with radioactive minerals if they are not radioactive themselves. Thus, the principal tool used in prospecting for rare earths is a radiation detector such as a geiger counter or scintillometer. Airborne radiometric surveys are also useful in finding larger placer deposits.
As mentioned above, rare earth minerals are difficult to identify. Most rare earth minerals are undifferentiated and distinguishable to only the most sophisticated mineralogist in the field. Radioactivity of many rare earth minerals is a useful aid in identifying the possible presence of rare earth minerals. However, the radioactivity of an ore sample provides the prospector with no specific information concerning whether the sample is a radioactive rare earth mineral or some other radioactive mineral. The actinides, as well as many other elements which are not rare earths, are also radioactive. Thus, even if an ore sample is radioactive, it is still necessary to have that sample analyzed to determine whether it contains rare earth elements.
Currently, the only known method for accurately detecting rare earth minerals is to have the ore sample analyzed in a laboratory. These laboratory procedures generally are used to identify the presence of specific rare earth elements. Such procedures are sophisticated and require expensive equipment to perform x-ray diffraction, x-ray optical fluorescence, atomic absorption or other chemical or spectroscopic analysis of the sample.
Neither spectroscopic nor x-ray analysis procedures for detecting rare earth minerals can presently be conveniently carried out in the field. Similarly, chemical analysis cannot be conveniently performed in the field without somewhat complicated equipment and burdensome processes. Thus, none of the current methods for detecting rare earth elements can be conveniently adapted for routine analysis or assay of rare earth mineral samples in the field.
In summary, rare earth elements obtained from rare earth minerals are playing an increasingly important role in today's society. The sophisticated and complex analytical techniques for identifying the presence of rare earth elements may not be suitably adapted for detecting the presence of rare earth minerals in the field. It will be appreciated that it would be a significant advancement in the art to provide methods for detecting rare earth minerals which can be performed in the field.
Additionally, it would be a significant advancement in the art to provide methods for detecting rare earth minerals which provide quick and accurate results.
It would be another advancement in the art to provide methods for detecting rare earth minerals which do not require sophisticated chemicals, processes, or equipment.
It would be a further advancement in the art to provide methods for detecting rare earth minerals which use relatively safe and self-neutralizing reagents.
It would be yet another advancement in the art to provide methods for detecting rare earth minerals which do not require the use of radioactivity detectors.
Such methods for detecting rare earth minerals are disclosed and claimed herein.